1. Field of the Invention
The present invention relates to a semiconductor device used for high-power control, and more in particular to a semiconductor device capable of reducing an on-state voltage drop.
2. Description of the Related Art
In recent years, the Si MOSFET has been widely used as a semiconductor device for high-power control. This MOSFET is a unipolar device and has various advantages including high speed, ease to control, etc.
FIG. 1 is a diagram schematically showing a configuration of this type of MOSFET. This MOSFET comprises an n-type substrate 301 constituting an n-type drain layer, an n-type base layer 302 formed on the n-type substrate 301, a plurality of p-type base layers 303 selectively formed by diffusion in the surface of the n-type base layer 302, and n-type source layers 304 selectively formed in the surface of each p-type base layer 303.
A gate electrode 306 is formed through a gate insulating film 305 in a region extending from a first p-type base layer 303 and an associated n-type source layer 304 through the n-type base layer 302 to a second p-type base layer 303 and an associated n-type source layer 304. Also, a source electrode 307 is formed on each set of the p-type base layer 303 and the n-type source layer 304 in such a position as to hold a gate electrode 306 between each two sets.
Further, the surface of the n-type substrate 301 far from the n-type base layer 302 is formed with a drain electrode 308.
This MOSFET is a unipolar device contributing to the conduction of a type of carriers. If the resistance of the MOSFET is to be reduced, therefore, the impurities concentration of the n-type base layer 302 is required to be increased to decrease the resistivity thereof and also to decrease the thickness of the n-type base layer 302.
With the increase in the impurities concentration of the n-type base layer 302, however, the maximum intensity value of the electric field formed just under the p-type base layer 303 with the MOSFET off increases. As a result, in the MOSFET, the impurities concentration in the n-type base layer 302 is required to be suppressed in order for the maximum value of the electric field intensity not to exceed a critical level of the electric field intensity of the n-type base layer 302. Also, the breakdown voltage of the MOSFET is determined by the total amount of impurities in the n-type base layer 302. When the breakdown voltage is to be increased, therefore, the n-type base layer 302 is thickened. In the MOSFET of high breakdown voltage, therefore, the on-state voltage drop rapidly rises.
In short, in this type of MOSFET, an improved breakdown voltage and a lower on-resistance are desired. For increasing the breakdown voltage, the thickness W of the n-type base layer 302 is increased or the carrier density N of the n-type base layer 302 is reduced.
These methods, however, as shown by solid line in FIG. 2, have been theoretically found to undesirably increase the on-resistance about two orders of magnitude before improving the breakdown voltage an order of magnitude. In other words, the solid line in FIG. 2 indicates that there is a theoretical limit determined from the physical property of Si and that the MOSFET with a high breakdown voltage has a high on-resistance as compared with the IGBT or the like.
Now, explanation will be made sequentially about the bipolar transistor (hereinafter referred to as the BJT) and the IGBT used for power control like the MOSFET.
FIG. 3 is a sectional view schematically showing a configuration of a bipolar transistor. This BJT is such that an n-type base layer 312 is formed on an n-type substrate 311 as an n-type collector layer. The surface of the n-type base layer 312 is selectively formed with a p-type base layer 313 by diffusion. The surface of the p-type base layer 313, in turn, is selectively formed with an n-type emitter layer 314. A base electrode 315 is formed on the p-type base layer 313. An emitter electrode 316 is formed on the n-type emitter layer 314.
Also, the surface of the n-type base layer 311 far from the n-type base layer 312 is formed with a collector electrode 317.
In this BJT, a large proportion of the current flowing in the n-type base layer 312 is due to electrons. Like in the MOSFET, therefore, the on-state voltage drop rapidly increases undesirably with the increase in the breakdown voltage.
On the other hand, an attempt has been made to reduce the on-state voltage drop by making a high-resistance n-type base layer a high injection state like the IGBT.
FIG. 4 is a sectional view schematically showing a configuration of an IGBT. In this IGBT, a plurality of p-type base layers 322 are selectively formed in the surface of a high-resistance n-type base layer 321. The surface of each p-type base layer 322 is formed selectively with n-type source layers 323 by diffusion. A gate electrode 325 is formed through a gate insulating film 324 in a region extending from a first p-type base layer 322 with an n-type source layer 323 to a second p-type base layer 322 through the n-type base layer 321. Also, a source electrode 326 is formed on each p-type base layer 322 with the n-type source layers 323 in such a manner that each two adjacent source electrodes 326 hold a gate electrode 325 therebetween. Also, a drain electrode 328 is formed on the reverse side of the n-type source layer 321 through a p-type drain layer 327.
Upon application of a positive voltage to the gate electrode 325 of this IGBT, the portion of the p-type source layer 322 under the gate electrode 325 is formed with an n-type inversion layer thereby to short the n-type base layer 321 and the n-type source layer 323 to each other. Consequently, electrons are injected into the n-type base layer 321, and holes are injected from the p-type drain layer 327 in accordance with the amount of electrons thus injected, so that the n-type base layer 321 becomes a high injection state thereby to turn on the IGBT. When the IGBT is in on state, the n-type base layer 321 becomes a high injection state. Even when the n-type base layer 321 has a high resistivity, therefore, the resistance of the IGBT is low.
A current does not flow in the IGBT, however, unless a voltage higher than the diffusion potential difference between the n-type base layer 321 and the p-type drain layer 327 is applied between the source electrode 326 and the drain electrode 328. As shown in FIG. 5, therefore, the on-state voltage drop of this IGBT is higher than that of the MOSFET, and a conduction loss of the IGBT is increasing, when the current value is low.
More specifically, with the increase in breakdown voltage, the MOSFET or BJT poses the problem of an on-state voltage drop rising with higher rapidity. The IGBT, on the other hand, has the problem of a larger conduction loss caused with a lower current state.